This invention relates generally to communications and, more particularly, to high-speed data communications systems.
Plain Old Telephone Service (POTS) is typically deployed to individual subscribers over a twisted pair of wire. Today, in addition to voice services, more and more subscribers want high-speed data access to, e.g., the Internet, over this twisted pair. One technology that increases the transmission capacity over a twisted pair is Asymmetric Digital Subscriber Loop (ADSL). One version of ADSL increases the bandwidth of the twisted pair up to 1.1 Mhz (megahertz), which provides transmission capabilities up to 9 Mbps (millions of bits per second).
ADSL allocates different amounts of bandwidth between upstream communications and downstream communications (hence the term xe2x80x9casymmetricxe2x80x9d), with upstream communications having less bandwidth than downstream communications. In this context, there are different strategies for specific bandwidth allocation and different modulation methods available. For example, in the upstream direction, i.e., from a subscriber""s consumer premises equipment (CPE) to a central office (CO) (or local exchange carrier (LEC)) the upstream channel may have an allocated bandwidth from 25 Khz (kilohertz) to 138 Khz; while in the downstream direction, i.e., from the CO to the CPE, the downstream channel may have an allocated bandwidth from 138 Khz to 1.1 Mhz. (The POTS voice channel (0 to 4 Khz) is unaffected by ADSL.). In this example, the upstream channel and downstream channel are disjoint and also adjacent. However, ADSL systems can be constructed where the upstream channel partially overlaps with the downstream channel. While this provides more bandwidth for the downstream signal, this also requires the use of echo cancellation techniques. Turning to modulation methods, carrierless amplitude phase (CAP) modulation or Discrete Multi-Tone (DMT) modulation can be used. (DMT is a form of orthogonal frequency division multiplexing (OFDM).)
One standard for ADSL transmission is ANSI T1.413. This standard specifies the use of DMT modulation, which utilizes multiple carriers (also sometimes referred to as subcarriers) for conveying information. In DMT modulation, the allocated frequency range is divided into K carrier channels, K greater than 1, each carrier channel separated by approximately 4 Khz. In such an approach, a DMT-based ADSL system transmits what is referred to as xe2x80x9cmulti-tone symbolsxe2x80x9d or xe2x80x9cDMT symbols.xe2x80x9d
One problem in an ADSL DMT-based system is the complexity, and resulting system cost, of echo cancellation if there is an overlap between the upstream channel and the downstream channel. This complexity is due to the fact that each carrier may be interfered with by a plurality of the other carriers. Consequently, it is cheaper to offer ADSL systems in which the upstream channel and the downstream channel are disjoint.
Another problem in an ADSL DMT-based system is that there may be intersymbol interference (ISI), i.e., adjacent DMT symbols interfering with each other. To reduce the effect of ISI, the concept of xe2x80x9ccyclic extensionxe2x80x9d is used. In cyclic extension, a DMT symbol is partially, and cyclically, extended in both directions. A conceptual illustration is shown in FIG. 1 for a single carrier at a receiver. The DMT symbol is represented by, e.g., the phase of carrier 10. Cyclic extension occurs before and after the DMT symbol. Essentially, carrier 10 is allowed to extend in both directions as represented by cyclic extension 1 (CE1) and cyclic extension 2 (CE2), which may or may not be equal to each other. CE1 and CE2 are a form of overhead in DMT transmission and are also known in the art as the prefix and the postfix, respectively. It should be noted that in practice an ADSL DMT transmitter simply adds a single cyclic extension, CE, to the DMT symbol, as shown in FIG. 1; while the receiver adjusts the sampling process such that, two apparent cyclic extensions, a CE1 and CE2 result, where CE=CE1 and CE2. As used herein, the term extended DMT symbol includes the DMT symbol plus at least one cyclic extension. The value of CE is adjusted as a function of the span of an estimated channel impulse response.
We have observed that in a DMT-based system using the above-mentioned cyclic extensions and having disjoint and adjacent upstream and downstream channels, interference may still occur between different carriers of the upstream channel and the downstream channel. However, we have realized that synchronization of DMT symbol transmissions reduces this interference. Therefore, and in accordance with the invention, a multi-tone transmitter is synchronized with a far-end multi-tone transmitter. Additionally, we have realized that such synchronization simplifies the design of an echo canceler, which can be used in multi-tone systems where the upstream channel and downstream channel overlap. (Even in the case of using disjoint and adjacent upstream and downstream channels, echo cancellation may also be needed due to the roll-off in the excess bandwidth expansion in the upstream channel if the upstream signal is generated using an inverse fast Fourier transform of a size equal to the number of carriers in the signal.)
In an embodiment of the invention, an ADSL DMT system has disjoint and adjacent upstream and downstream channels. During the training phase of an ADSL connection, an ADSL DMT transmitter first determines a round trip propagation delay by transmitting a ranging signal to a far-end ADSL endpoint. During the subsequent communications phase, the ADSL transmitter synchronizes transmission of DMT symbols to a reference clock. In addition, the cyclic extensions of each DMT symbol are increased as a function of the propagation delay.
In a second embodiment of the invention, an ADSL DMT system has an upstream and downstream channel that partially overlap. During the training phase of an ADSL connection, an ADSL DMT transmitter first determines a round trip propagation delay by transmitting a ranging signal to a far-end ADSL endpoint. During the subsequent communications phase, the ADSL transmitter synchronizes transmission of DMT symbols to a reference clock. In addition, the cyclic extensions of each DMT symbol are increased as a function of the propagation delay. An ADSL receiver includes a single tap echo canceler for each carrier in that portion of bandwidth where the upstream and downstream channels overlap.